From DE 100 12 448 A1 a method of distributing the braking force in vehicles is known, which checks whether wheel brakes are overheated or at risk of overheating. If so, the wheel brakes are alternately loaded with braking force. The effect achieved by this is that in the periods, during which one of the wheel brakes is not loaded with braking force, this wheel brake may cool down. In order to allow emergency braking operations also for wheel brakes that are overheated or at risk of overheating, the wheel brakes are loaded, not in the described manner, but in a conventional manner with braking force when vehicle decelerations are specified, which exceed a limit value. Here, overheating or a risk of overheating of wheel brakes exists if the temperature of the wheel brakes has exceeded a predetermined limit range or limit value or lies within a predetermined temperature range, the exceeding of which counts as overheating. For this purpose, it is provided that the temperatures of the wheel brakes are directly detected by temperature sensors or determined with the aid of a temperature model. Given the use of a temperature model, it is provided that, as a basis, quantities are used, which indicate hydraulic pressures in the case of hydraulic brakes, current input in the case of electromechanical brakes, brake application time and the like.
According to DE 44 18 768 A1, and corresponding U.S. Pat. No. 5,524,974 A1, both of which are incorporated by reference herein, the temperature of a brake surface of a motor vehicle wheel is calculated indirectly from the wheel rotational speed and brake condition data. During vehicle operation, the brake surface temperature is cyclically re-determined, wherein in each cycle the current brake surface temperature is reduced by a first value to, at least, a preset minimum temperature. It is further provided that upon an activation of the wheel brake the current brake surface temperature in each cycle is increased in each case by a second value, which is determined in dependence upon data resulting from detected wheel rotational speeds and/or from quantities derivable therefrom. The quantities derivable from the data for detected wheel rotational speeds comprise the vehicle velocity and a vehicle deceleration because of the braking effect. For determining the second value, which is used to increase the current brake surface temperature, the deceleration energy, which is converted into thermal energy and supplied as such to the wheel brake, is calculated, which is proportional to the product of vehicle velocity and vehicle deceleration. To prevent damage of the brake surfaces, regulating interventions of the control system of the brake system (e.g. ABS, electronic traction system) are in dependence upon the brake surface temperatures automatically interrupted, switched off or stopped for the time being in order to avoid excessive heating of the brake surfaces. This may lead e.g. in a situation, which the vehicle driver is no longer able to control on his own and which would require assistance in the form of a regulated operation of the brake system, to this assistance not being made available owing to excessive brake surface temperatures. This in turn may result in damage to the vehicle that is more serious than potential damage of brake surfaces caused by overheating.
For determining a temperature of a brake disk of a motor vehicle, from DE 38 13 514 A1 it is known to use a transmitter, which responds to changes of temperature and is disposed in the vicinity of the brake disk in order to detect its thermal radiation. For this it is necessary to dispose a suitable, temperature-sensitive transmitter at each brake disk that is to be monitored. The fact that the transmitters are disposed at a distance from the brake disks may lead to incorrect temperature determination, e.g. because of road spray or fouling of the sensor or detection by the transmitter of thermal radiation given off by other components of the vehicle.
DE 199 43 352 A1 discloses an apparatus and a method of determining the temperature of braking elements provided on a vehicle, whereby the instantaneous kinetic energy of the vehicle is calculated and compared to a previous value of the kinetic energy of the vehicle. Upon a decrease of the kinetic energy of the vehicle, from this decrease the thermal energy supplied to the brake elements is calculated and from this the temperatures of the brake elements are calculated. To determine the kinetic energy of the vehicle, the mass of the vehicle including the payloads and the instantaneous vehicle velocity are used. In order, when calculating the temperatures of the brake elements, to take account of external influences, such as e.g. the gradient of the road, the condition of the road surface and/or the resultant friction of the vehicle wheels on the road surface, the laden state of the vehicle, the variation of the drag coefficient of the vehicle caused e.g. by a roof load, and the like, in each case corresponding quantities are determined, e.g. by means of additional sensors, and used when calculating the temperatures of the brake elements.
From DE 43 16 993 A1 it is known to determine the temperature of brake disks of a vehicle brake system indirectly, i.e. without the aid of corresponding sensors. For this purpose, during a braking operation quantities characterizing the energy balance of the motor vehicle are acquired, such as e.g. the vehicle weight, the vehicle velocity and vehicle acceleration, wheel peripheral speeds and wheel accelerations and the road gradient. From these quantities, the frictional work to be done by each brake lining and the friction power are calculated. The temperature of the brake disks is then determined in each case from the corresponding calculated frictional work and friction power, the active brake disk mass and the specific thermal capacity of the brake disk material. The determined brake disk temperature is used to obtain information about the load state of the brake system and about whether a specific braking sequence, i.e. a chronological order of braking forces at the brake disks is capable of keeping the brake disk temperature within a desired range. Indications of the further use of such information are not to be found in this document.
From DE 40 20 693 A1 it is known to calculate temperature changes of brakes for short preset intervals from the sum of the square of the vehicle velocity and the square of the vehicle deceleration, multiplied in each case by a vehicle-specific constant. Should a temperature rise lead to the exceeding of a limit value for the brake temperatures, a warning lamp switches on. Furthermore, the engine torque may be reduced in order to achieve a speed, at which stopping without brakes or braking without brake overheating is possible. This automatic speed reduction may lead to dangerous driving situations, e.g. when travelling on motorways. Alternatively, it is provided that in the event of excessive brake temperatures the antiskid device of the vehicle is switched off. This has the drawback that, whilst damage to the brakes caused by excessive temperatures may be avoided, this is offset by the fact that more serious damage of the vehicle may occur as a result of the antiskid device being switched off.